FIG. 11 to FIG. 16 show a rotary connector disclosed in the following Patent Document 1.
The rotary connector 100 is provided with a first connector housing 110 and a second connector housing 120. The first connector housing 110 includes a body portion 111 having a circular outer circumference shape in cross section. The second connector housing 120 includes a cylinder portion 121 into which the body portion 111 is fitted.
Connection groove portions 112 are formed at two circumferentially separate places in an outer circumference surface of the body portion 111 of the first connector housing 110. Each of the connection groove portions 112 includes an axial groove 112a and a circumferential groove 112b. The axial groove 112a is arranged to extend along a central axis O1 of the body portion 111 from a distal end portion of the body portion 111. The circumferential groove 112b is arranged to extend circumferentially along the body portion 111 from a distal end of the axial groove 112a. A large diameter portion 114 which has a substantially column shape concentric to the body portion 111 and which has a larger diameter than the body portion 111 is formed in a proximal end of the body portion 111. As shown in FIG. 11 and FIG. 13, a lock pin 116 is provided in a step face 115 formed in a borderline between the body portion 111 and the large diameter portion 114.
As shown in FIG. 13 and FIG. 14, the lock pin 116 is retained in a pin support hole 115a made in the step face 115. The pin support hole 115a is formed along the central axis O1 of the body portion 111. The lock pin 116 is slidably fitted into the pin support hole 115a. As shown in FIG. 14, coming off of the lock pin 116 received in the pin support hole 115a can be prevented by a sleeve 117 pressed into the pin support hole 115a. In addition, the lock pin 116 is urged by a compression coil spring 118 in a direction to protrude from the pin support hole 115a. The compression coil spring 118 is disposed in a compressed state inside the pin support hole 115a. 
Due to an urging force of the compression coil spring 118, normally, the lock pin 116 is retained in a state in which a distal end of the lock pin 116 protrudes from the pin support hole 115a. The lock pin 116 can be displaced in a state in which the total length of the lock pin 116 is stored inside the pin support hole 115a due to compression of the compression coil spring 118.
In the second connector housing 120, connection protrusions 122 are provided protrusively at two places in an inner circumference of a distal end of the cylinder portion 121, and a pin engagement groove 126 is formed in an outer circumference of the distal end of the cylinder portion 121.
The connection protrusions 122 are protrusions engaged with the connection groove portions 112 of the first connector housing 110. When the body portion 111 is fitted into the cylinder portion 121 in a state in which the positions of the connection protrusions 122 are aligned with the positions of the axial grooves 112a of the body portion 111, the connection protrusions 122 enter the axial grooves 112a and arrive at the distal ends of the axial grooves 112a. When the first connector housing 110 and the second connector housing 120 are rotated relatively to each other in this state, the connection protrusions 122 enter the circumferential grooves 112b to thereby restrict the connector housings from being displaced axially relatively to each other.
Incidentally, when the cylinder portion 121 and the body portion 111 are fitted to each other axially, the lock pin 116 is pushed into the pin support hole 115a by a front end of the cylinder portion 121 such that the total length of the lock pin 116 is stored inside the pin support hole 115a. 
The pin engagement groove 126 is a groove into which the lock pin 116 stored inside the pin support hole 115a plunges as soon as the connection protrusions 122 arrive at distal ends of the circumferential grooves 112b due to relative rotation between the connector housings. When the lock pin 116 plunges into the pin engagement groove 126, the connector housings are restricted from rotating relatively to each other such that the connection state between the connector housings is locked.
In the case of the rotary connector 100 described in Patent Document 1, the lock state between the connector housings can be released in the following manner. That is, as shown in FIG. 15, a special tool 130 is wedged into a space in the pin engagement groove 126 to thereby store the lock pin 116 inside the pin support hole 115a, as shown in FIG. 16. Then, an operation is performed to rotate the connector housings relatively to each other.